In Ethernet (registered trademark) network, there is a case of taking a ring network topology in which bridge apparatuses are connected in a ring-shape. This topology enables a physical redundant route to be provided between arbitrary bridge apparatuses on the network and enables network reliability to be enhanced.
In the Ethernet (registered trademark) network having the ring network topology, however, such a phenomenon occurs that a data frame is permanently circulated with the result that normal communications can not be performed. An STP (Spanning Tree Protocol), a ring protocol, etc are utilized for preventing this type of phenomenon. The STP is a protocol for providing a logical route tree by transferring and receiving a control packet named BPDU (Bridge Protocol Data Unit) between the bridge apparatuses on the basis of given priority levels. According to the STP, on the occasion of generating the tree, a port of the bridge on an unselected route is logically blocked. With this scheme, a data frame relay route between the arbitrary bridge apparatuses is uniquely determined.
FIG. 12 is a view illustrating an example of an operation of the ring protocol. FIG. 12 depicts a ring network 1 in which bridges 11, 12, 13 and 14 are connected in the ring-shape. The ring protocol is implemented in the ring network 1, where a port of the bridge 12, with which the bridge 11 connects, is blocked. The data frame sent to a user B from a user A is thereby forwarded uniquely on a route such as the bridge 12-the bridge 14-the bridge 13. On the other hand, if a fault occurs within the network, the communications resume on a route where any fault does not occur by dynamically changing the blocking point.
By the way, a VLAN (Virtual Local Area Network) technology is exemplified as a technology of configuring the network like this as a virtual network that is not limited by a physical topology. In the VLAN, a plurality of virtual networks (broadcast domains) are formed by segmenting a broadcast domain owing to a function of a switch device (a layer-3 switch, a layer-2 switch, etc). Further, the VLAN is classified into several categories such as a port-base VLAN and a tag VLAN, in which virtual network configuring methods are different. The tag VLAN is of a technology standardized by IEEE802.1Q, wherein one single port of the switch device can belong to a plurality of virtual networks by adding a 4-byte VLAN tag field to an Ethernet (registered trademark) frame.
FIG. 13 is a diagram illustrating a frame format of the tag VLAN specified by IEEE802.1Q. The VLAN tag field is added to between a source address field and a type field, and contains a tag protocol identifier (TPID: Tag Protocol IDentifer) and tag control information (TCI: Tag Control Information). A 12-bit VLAN identifier (which will hereinafter be abbreviated to VLAN ID) is set in the TCI. The VLAN ID is information for identifying the segmented virtual networks, in which numeral values of 1 through 4094 are specified as usable values.
In a ring network where the tag VLAN is implemented, a technique of periodically confirming normality of forwarding a data frame involves using a technique of periodically transmitting and receiving a conduction confirmation frame. As for the VLAN ID of this conduction confirmation frame, there are a technique (conventional technique 1) of setting the VLAN ID specially prepared for the conduction confirmation and a technique (conventional technique 2) of setting an VLAN ID being used.
Note that the conventional techniques related to the invention of the present application are disclosed in the following documents. Patent document 1 given below discloses a packet forwarding method for actualizing an uninterrupted switching function by generating a copy of the packet when a fault occurs, transferring those separately on different routes and forwarding any one of the two packets on the receiving side. The Patent document 1 is Japanese Patent Laid-Open Publication No. 2005-102157.